testClassesDirectory not added to classpath

When I change the testClassesDirectory, these classes don't appear to be added to the classpath.
I want to be able to compile test classes to the standard test-classes directory, then modify the classes and put the modified classes into another directory. Currently when I try to do this, the original test-classes are being picked up from the class path, and not the modified classes.

Ideally the original test-classes directory should not be included in the classpath if I specify a value for testClassesDirectory.